1. Field of the Invention
This invention relates to high density lead frames and methods for plastic injection molding. More particularly, the present invention relates to high density lead frames and methods for plastic injection molding that minimize waste of both the lead frame material and the plastic injection molding material, while increasing manufacturing productivity.
2. Description of the Related Art
Semiconductor integrated circuits are usually mounted on lead frames, each circuit thereafter being encapsulated in a plastic package formed by an injection molding process. FIG. 1 shows an example of a first generation of a lead frame assembly 100. The lead frame assembly 100 includes a first lead frame 110 and a second lead frame 140. The first and the second lead frames 110, 140 are connected by an injection molding assembly 105. The injection molding assembly 105 includes a plurality of culls 115 through which the molding compound (not shown) travels. The molding compound is injected into each of the culls 115 and travels through pairs of subrunners 120 formed during the molding process to the first and second lead frames 110, 140. The subrunners 120 are shaped like tunnels through which the molding compound flows. The molding compound, initially in powder form, is heated to about 175.degree. C., at which temperature the powder compound becomes a liquid. The hot liquid molding compound is then injected through an opening in the culls 115 and forced to flow through the subrunners 120. The molding compound flows through the subrunners 120 until the gates 125, also generally tunnel-shaped, are reached, the gates 125 guiding the molding compound to the mold location where the individual packages 130 are to be formed. The molding compound then forms the individual packages 130, encapsulating the integrated circuits therein in individual packages, leaving the leads 132 protruding therefrom. In FIG. 1, the packages 130 can be seen to be oriented with their leads 132 oriented perpendicularly to the long axes 150 of the first and second lead frames 110, 140.
As shown in FIG. 1, the first and the second lead frames 110, 140 each support five pairs of packages 130, each pair of packages 130 having been supplied with molding compound by a single subrunner 120 and a gate 125. The lead frame assembly 100 of FIG. 1 thus creates twenty such packages 130, ten packages 130 in each of the first and second lead frames 110, 140. The packages 130 are then trimmed from the first and second lead frames 110, 140, the leads 132 appropriately bent and shaped, and the packages 130 separated from one another in a simulation process.
The first and second lead frames 110, 140 are formed of copper or of a copper alloy. After the simulation process, all but the packages 130 themselves are discarded. Indeed, the remaining portions of the first and second lead frames 110, 140 are discarded, as is the injection molding assembly 105, including the culls 115, the subrunners 120 and the gates 125. Because of the design and layout of the lead frame assembly 100, a great deal of lead frame material and injection molding assembly 105 is thrown out, thus significantly adding to the cost of the end product. Moreover, because of the low density of packages 130 on each of the first and second lead frames 110, 140, labor costs are high and manufacturing yield is low, both also contributing to increased unit cost of the packages 130.
In an effort to reduce material waste and labor costs and to increase manufacturing yield, the lead frame assembly 200 of FIG. 2A has been developed. FIG. 2A shows the lead frame assembly 200 with a plurality of packages 230 molded thereon. The lead frame assembly 100 of FIG. 2A includes a first and a second lead frame 210, 240 joined by an injection molding assembly 205. The structure of the injection molding assembly 205 is similar to that of its counterpart 105 in FIG. 1, and includes a plurality of centrally located culls 215 through which the molding compound (not shown) is injected. The molding compound is injected into each of the culls 215 and travels through pairs of subrunners 220 formed by the mold to the first and second lead frames 210, 240. The subrunners 220 are generally tunnel-shaped structures through which the molding compound flows. The molding compound flows through the subrunners 220 until it reaches the gates 225 (also generally tunnel-shaped), the gates 225 guiding the molding compound to the location where the individual packages 230 are to be formed. The molding compound then fills these locations to encapsulate the integrated circuits therein to form the individual packages 230, leaving the leads 232 protruding therefrom. In FIG. 2A, the individual packages 230 can be seen to be again oriented with their respective leads 232 oriented generally perpendicularly to the long axes 250 of the first and second lead frames 210, 240.
The lead frame assembly 200 shown in FIG. 2A is designed to support a higher number of packages 230 than its predecessor lead frame assembly 100 of FIG. 1. Indeed, each subrunner 220 of FIG. 2A is connected to three gates 225, each gate 225 supplying a pair of packages 230. Therefore, each subrunner 220 supplies molding compound to 6 separate packages 230. The lead frame assembly 200 depends upon a network of subrunners disposed squarely within the confines of the lead frames 210, 240 to form pairs of packages 230.
The first and second lead frames 210, 240 are formed of copper or of a copper alloy. After the simulation process referred to above, all but the packages 230 are discarded. Indeed, the remaining portions of the first and second lead frames 210, 240 are discarded, as is the injection molding assembly 205, including the cuffs 215, the subrunners 220 and the gates 225 joining pairs of packages 230. Because of the architecture of the lead frame assembly 200, much lead frame material and injection molding assembly 205 is discarded, thus again significantly adding to the cost of the end product. Moreover, because of the low density of packages 230 on each of the first and second lead frames 210, 240, labor costs are high and manufacturing yields low, albeit not as high nor as low, respectively, as with the lead frame assembly 100 discussed relative to FIG. 1.
FIG. 2B shows a lead frame 210, before the packages 230 (shown in FIG. 2A) are molded thereon. As shown in FIG. 2B, the die attach pads 231, the pads onto which the semiconductor die or dice is to be attached, are oriented in such a manner that the leads 232 extend generally perpendicularly relative to the long axis 250 of the lead frame 210. As shown in both FIGS. 2A and 2B, the die attach pads 231 and their corresponding packages 230 shown in FIG. 2A are organized in rows and columns. Indeed, the rows of die attach pads 231 are parallel to the long axis 250 of the lead frame 210 (and 240 in FIG. 2A), whereas the columns thereof are perpendicular thereto. Each column of die attach pads 231 is separated from its next adjacent column by a space called a street, shown in FIG. 2B at reference numeral 260. The streets 260 may be perforated, as shown in FIG. 2B, to reduce the amount of lead frame material (e.g., copper) needed. The streets 260 include notches 265 for the gates 225 through which the liquid molding compound emerges to form the packages 230.
The streets 260 provide the space and necessary support for the subrunners 220 and the gates 225. To accommodate the network of subrunners 220 and gates 225 shown in FIG. 2A, the die attach pads 231 and their corresponding packages 230 must necessarily be separated by a substantial space. Such space inherently precludes a greater unit density on the lead frames 210, 240 and leads to manufacturing inefficiencies. Indeed, after the simulation process is carried out, the injection molding assembly 205 and the remaining lead frame materials are simply discarded, at great cost to the manufacturer and ultimately to the consumer of the end devices. Moreover, the generation of such a quantity of waste materials may have an adverse affect upon the environment, unless disposed of properly and/or recycled to the extent possible.
What is needed, therefore, is a high-density lead frame for plastic injection molding that increases the density of packages on the lead frames. What is also needed is a lead frame and injection molding assemblies and methods that minimize the amount of material that must be discarded after the simulation process is carried out. What is also needed is a novel lead frame design that minimizes some of the environmental impacts of the manufacture of semiconductor devices.